Usually, an electric power steering device (EPS) that has a motor as a drive source includes a motor rotation angle sensor that detects a rotation angle of the motor and a torque sensor that detects steering torque. The torque sensor is configured by a rotation sensor that outputs a sensor signal based on torsion of a torsion bar provided in the middle of a steering shaft. A control device of the EPS controls the motor to apply an assist force to a steering system based on a sensor signal. Therefore, detecting stably and in high precision a motor rotation angle and steering torque is one of important tasks of the EPS.
Patent Document 1 discloses a configuration for improving reliability of a torque sensor by avoiding an electric contact portion from the torque sensor, by using a noncontact-type magnetism detecting element in a sensor element. Based on this configuration, the number of sensor elements can be easily increased without increasing the magnitude of the torque sensor. Detection precision of steering torque also improves based on multiplexing of sensor signals. On the other hand, when one of the sensor elements is out of order, application of an assist force can be continued by detecting steering torque based on sensor signals generated by residual sensor elements (assist continuation control).
Patent Document 2 discloses a method for specifying in higher precision a sensor element that is out of order when any one of sensor elements of a plurality of systems becomes abnormal. By specifying the sensor element that is out of order, a situation in which assist continuation control of the EPS is possible can be expanded.
However, to enjoy the benefit of multiplexing sensor signals, at least two sensor signals are necessary, as a matter of course. Further, particularly because a magnetic sensor element has a variation in temperature characteristics, a correction process using multiple sensor signals is essential for detection in high precision. Therefore, conventionally, as described in Patent Document 2, after only one sensor signal remains, assist continuation control needs to be executed such that an assist force is gradually reduced (gradual reduction) using the residual sensor signal so as to promptly stop application of the assist force.